1. Field of the Invention
This invention relates to a temperature control device for controlling the surface temperature of a member heated by a heating source.
2. Description of the Related Art
As shown in FIG. 11, a conventional fixing roller 16 of a thermal fixing device in an electrophotographic apparatus incorporates a heater 12, serving as a heating member. A thermistor 1, serving as a temperature detection means, is disposed in contact with the surface of the fixing roller 16. The heater 12 and the thermistor 1 are connected to a temperature control device (not shown), comprising a CPU and the like, so as to maintain the temperature of the surface of the fixing roller 16 at a predetermined temperature. An explanation will now be provided of a constant temperature control method with reference to FIGS. 11 and 12.
The temperature of the fixing roller 16 is detected by the thermistor 1. If the detected temperature is lower than a preset temperature (hereinafter after termed a threshold temperature), a heater drive circuit starts to heat the fixing roller 16 by supplying the heater 12 with current (step 301 in FIG. 12). When the temperature of the fixing roller 16 reaches at least the threshold temperature (represented by Ts in FIG. 12), current supply for the heater 12 is stopped (from step 302 to step 303 in FIG. 12). As shown in FIG. 13, the temperature of the fixing roller 16 first rises to a peak temperature above the threshold temperature, and then, after the heater steps, starts to descend and drops below the threshold. When the temperature of fixing roller 16 becomes lower than the threshold, the process returns to step 301, where the roller 16 starts to be heated. By repeating such a control, the surface temperature of the fixing roller 16 is maintained within a predetermined variance from the threshold.
In the above-described conventional control, however, due to a time lag produced between the actual temperature of the fixing roller and the detected temperature, and due to a time lag in heat transfer to the surface of the fixing roller, the surface temperature of the fixing roller continues to rise to a considerably high temperature even after stopping current supply for the heater, producing a large temperature ripple, as shown in FIG. 13.